1. Field of the Invention
The present invention relates to optical transmission. More particularly, the present invention relates to an Ethernet passive optical network (Ethernet-PON) for providing high-volume, high-speed data services and real-time broadcast/image services to subscribers.
2. Description of the Related Art
Data transfer rates above 100 Mb/s are required to provide efficient high-volume, high-speed data services and real-time digital broadcast/image services to subscribers. However, for current network systems including those using a cable modem or xDSL to provide the high-volume it is impossible to achieve high-speed data services and real-time digital broadcast/image services since the systems have transfer rates of no more than 50 Mb/s. Thus, there has been a great deal of research in the area of providing a high-speed transmission network capable of providing high-volume, high-speed data services and real-time digital broadcast/image services. An optical network has been proposed as a potential solution for providing the backbone of such a high-speed transmission network. In particular, a passive optical network (PON) is attracting attention as an economically sound way to implement the optical network. There are various PONs, such as a PON based on Asynchronous Transfer Mode (ATM-PON), a PON based on Wavelength Division Multiplexing (WDM-PON) and a PON based on Ethernet (Ethernet-PON or EPON). An EPON-based FTTH (Fiber To The Home) system has been proposed and developed as a system capable of enabling high-speed optical transmission to general residences.
In general, the Ethernet-PON has been developed basically in order to accommodate communication data signals in an Ethernet network. Data transmission in the Ethernet-PON is performed in a manner such that Gigabit Ethernet signals are transmitted at 1.25 Gb/s from an optical line terminal (OLT) to an optical network terminations (ONTs) at a wavelength of 1550 nm, whereas Gigabit Ethernet signals are transmitted at 1.25 Gb/s from the ONTs to the OLT at a wavelength of 1310 nm. As a demand for broadcast services through the optical network increases, there is also an increased need to allow the Ethernet-PON to accommodate broadcast signals. To meet the need, an overlay broadcast accommodation system has been proposed in which broadcast signals are transmitted to the ONTs through a different wavelength for broadcast signals from the wavelength for communication data, as shown in FIG. 1. FIG. 1 illustrates the configuration of a general Ethernet-PON for integrating broadcast and communication functions. As shown in FIG. 1, the Ethernet-PON for integrating broadcast and communication includes an OLT (Optical Line Terminal) 100, a plurality of ONTs (Optical Network Terminals) 200-1 to 200-N, and a passive optical splitter 118. In addition, there are also optical cables for connecting the OLT 100 with the ONTs 200-1 to 200-N. The OLT 100 is a subsystem positioned between a service node and users, which receives broadcast and communication signals transmitted from broadcast and communication providers, and combines them into an optical signal after electro-optical conversion, and then subsequently transmits the optical signal. The ONTs 200-1 to 200-N are user-side devices for transferring information received from the OLT 100 to users.
In a more detailed explanation, the OLT 100 optically converts a broadcast signal received from a broadcast network through (optical/electrical) O/E and E/O (electric/optical) converters 115 and 116, and then transmits the converted signal after optically amplifying it through an EDFA (Erbium Doped Fiber Amplifier) 117. On the other hand, the OLT 100 receives communication data from an IP (Internet Protocol) network through an IP router 111, and processes it into an optical signal through an E-PON OLT function processor 112, and then transmits it through a transmitter 113. In addition, the OLT 100 receives data from the ONTs 200-1 to 200-N, and transmits it to the IP network through the IP router 111.
The ONTs 200-1 to 200-N receive broadcast signals through broadcast receivers 119-1 to 119-N, and transfer them to users through broadcast STBs (SetTop Box) 122-1 to 122-N. In addition, the ONTs 200-1 to 200-N receive communication data through receivers 120-1 to 120-N, and transfer it to users through E-PON ONT function processors 123-1 to 123-N.
Moreover, the ONTs 200-1 to 200-N receive communication data sent from users through the E-PON ONT function processors 123-1 to 123-N, and transmit it to the OLT 100 through the burst-mode transmitters 121-1 to 121-N.
In such a conventional Ethernet-PON system for accommodating broadcast signals as shown in FIG. 1, there is a requirement to provide the EDFA 117, a high-priced optical amplifier for broadcast signal amplification, in order to transfer analog broadcast signals from the OLT 100 to the ONTs 200-1 to 200-N. It should also be noted that even when the conventional Ethernet-PON system accommodates only digital broadcast signals no other signals such as analog broadcast signals, the E-PON system must be provided with the high-priced EDFA 117 if there are a large number of digital broadcast channels.
Further, since all broadcast channels are transmitted to each of the ONTs 200-1 to 200-N, it is also required that in order to receive the transmitted broadcast signals, the ONTs 200-1 to 200-N to include a high spec, high cost optical receiver, which has high reception sensitivity and excellent noise characteristics.
In the future, it is expected that users will demand not only digital broadcast services but also high-quality, real-time digital image services. However, there is still difficulty for the conventional Ethernet-PON to accommodate the high-quality, real-time digital image signals.
Moreover, there has been no suggestion as to how to transmit specific broadcast information from the ONTs 200-1 to 200-N to the OLT 100, in the conventional Ethernet-PON system, and it is thus difficult to implement bi-directional broadcasting functions that will be required in the future on conventional Ethernet-PON.